1. Field of the Invention
The present invention relates to alumina/zirconia ceramics and, particularly, to alumina/zirconia ceramics preferably used for various structural members, cutting tools, medical instruments and as a material for living bodies, and to a method of producing the same.
2. Description of the Related Art
In recent years, alumina or zirconia oxide ceramics have been extensively used as structural members where a high strength, abrasion resistance and corrosion resistance are required. In particular, alumina/zirconia ceramics containing alumina and zirconia at a predetermined ratio is drawing attention owing its high degree of strength stemming from the effect of fine crystalline particles, and there have been proposed alumina/zirconia ceramics of a variety of compositions.
For example, Japanese Unexamined Patent Publication (Kokai) No. 7-206514 and Japanese Unexamined Patent Publication (Kokai) No. 9-221354 are disclosing alumina/zirconia ceramics obtained by firing by using various assistants (SiO2, MgO and CaO), and teach that those containing alumina in an amount of not smaller than 70 mass % can be densely formed by firing at low temperatures and can, hence, be produced at decreased costs yet offering excellent abrasion resistance. However, the above alumina/zirconia ceramics containing alumina in large amounts exhibit good abrasion resistance accompanied, however, by a defect of small strength and insufficient hardness. Therefore, it has been desired to provide alumina/zirconia ceramics having improved strength, hardness as well as improved abrasion resistance.
Further, when the alumina/zirconia ceramics is used as a cutting tool, the cutting blade tends to be cut away and chipped off due to the lack of toughness and cannot be put to a practical use. Therefore, efforts have been made to improve the toughness by forming shape isotropic particles.
Japanese Unexamined Patent Publication (Kokai) No. 5-294718 teaches a highly tough alumina/zirconia composite material obtained by firing alumina and zirconia by adding SiO2 and at least one of BaO, SrO or CaO. That is, an assistant such as SrO is made present at the time of firing the alumina and zirconia, whereby Al2O3 crystals grow slenderly; i.e., Al2O3 particles assume a composition comprising slenderly grown crystals and excellent toughness is exhibited by the slenderly grown Al2O3 crystals. It has, however, been known that while the fracture toughness increases due to the formation of shape isotropic particles, the strength and hardness undergo a decrease. To further increase the fracture toughness, the shape isotropic particles must be grown more slenderly. However, the strength and hardness decrease with an increase in the size of the particles. That is, the alumina/zirconia ceramics disclosed in the above prior art exhibits improvement in the toughness due to the isotropic growth of alumina but has a bending strength of, for example, not larger than 1050 MPa, and the strength decreases due to the formation of the shape isotropic particles.